1. Field of the Invention
This invention relates to sliding electrical contact parts containing carbon material, typically graphite, and a metal or metal alloy designed to increase the electrical conductivity, typically copper. The invention relates more specifically to electrical contact brushes, in particular those used in starter motors.
2. Description of Related Art
Sliding electrical contact parts may contain additives of lead or antimony, in order to provide them with good electrical damping properties, a low coefficient of friction on a contact element, such as a commutator, and performances which remain constant in time. To improve the wear resistivity performances, the application no. FR 1 392 967 suggests the use of materials containing, apart from lead, manganese (Example 1) or tin+iron mixture (Example 2).
As the addition of lead is toxic and damaging to the environment, there is a high demand for contact parts containing little or no lead or antimony. However, it is not easy to replace lead or antimony by other metals known for their lubricating properties. Consequently, the European patent application no. EP 0 525 222 teaches to replace lead or antimony by tin or zinc, by taking measures designed to separate the copper from the zinc or tin additions, in order to prevent these elements from forming alloys.
However, in electrical devices that demand high current densities and friction speeds, such as automobile starter motors with high power weight ratios (especially flat commutator starter motors), the brushes, especially if they don't contain lead, and even of they contain no zinc, do not attain the required performances, especially in terms of life time and the stability in time of their characteristics. The applicant has therefore searched for solutions to this problem.